1. Field of the Invention (Technical Field)
The present invention relates to the control of electroslag remelting (ESR) furnaces, which are used to process metals and alloys to improve material quality. An electrode of the material to be processed is melted by the ESR process to produce a refined ingot. The process is characterized by the passage of current between the electrode and ingot through a molten, ionized slag. This slag is sufficiently heated by passage of current (Joule heating) so that the electrode is melted as it is immersed in the slag. The molten metal drips from the electrode through the slag and forms an ingot below.
2. Background Art
The ESR process is traditionally carried out commercially with alternating current (AC), and most control processors rely on RMS (root mean square) values of voltage and current to provide process information, while others gather information on the resistivity or conductivity of the AC circuit. Control of an ESR furnace is commonly achieved by regulating the voltage between the electrode and the ingot for a given current by moving the electrode either up or down in the slag. In most instances, the variation of voltage around the average voltage, known as voltage swing, is used to position the electrode. By regulating the voltage swing, the immersion depth of the electrode in the slag is attempted to be kept constant. However, it is known that the control strategies currently used in industry do not provide for accurate control of electrode immersion depth, resulting in imperfections in the ingot being produced.
Since ingot quality is tied to establishing and maintaining quasi-steady state melting conditions, the primary function of controllers currently in use is to keep the electrode immersed at as shallow a depth as possible because voltage swing only becomes significant under such a condition. Consequently, only at shallow immersion depths (approximately less than 25 mm) can voltage swing control become useful. However, because the voltage signal is inherently noisy, the controller operates over a relatively wide range of voltages which would fall within the range of the voltage swing. A controller based on this strategy has a major drawback: it can only control the process reliably when the electrode is very near the slag surface.
References relating to the general state of the art of electroslag remelting include the following: U.S. Pat. No. 4,027,233, to Shmakov et al., discloses a contactless inductance interface pickup, which in electroslag remelting may be used to detect the location of the interface between slag and air. U.S. Pat. No. 4,047,555, to Lamarque, discloses means for transporting consumable electrodes into and out of the slag in an electroslag remelting system. U.S. Pat. No. 4,108,235, to Paton et al., discloses an electroslag remelting apparatus incorporating a cooled mold assembly enabling the making of hollow ingots. U.S. Pat. No. 4,131,754, to Roberts, is concerned with providing for an initial electrode melt rate higher than a later, steady state melt rate in both electroslag and arc remelting furnaces. U.S. Pat. No. 4,262,159 discloses an electroslag remelting apparatus having a particular power arrangement. U.S. Pat. No. 4,291,744, to Medovar et al., discloses an electroslag remelting apparatus capable of simultaneously remelting a plurality of electrodes. U.S. Pat. No. 4,953,177, to Tommaney et al., discloses a system for controlling the atmosphere above the slag bath of an electroslag remelting furnace.
The references disclosing use of particular control parameters in analyzing and controlling electrode depth in electroslag remelting include: U.S. Pat. No. 3,890,457, to Fain et al., discloses a typical prior art device for controlling electroslag remelting. As noted at col. 1, lines 43-50, the device considers only the following parameters: remelting current, voltage, resistance of slag, and input power. U.S. Pat. No. 4,075,414, to Thomas, discloses an apparatus for regulating electrode immersion depth by measuring system resistance and the change of resistance over time. U.S. Pat. No. 4,194,078, to Thomas, suggests regulating immersion depth by measuring conductance of the slag blanket based upon a discovered linear relationship between the conductance and immersion depth. U.S. Pat. No. 4,303,797, to Roberts, discloses an electrode drive speed control system which, for electroslag remelting furnaces, determines the drive speed in part on the voltage across the slag bath or the magnitude of current through it (col. 3, lines 16-35). U.S. Pat. No. 4,476,565, to Rashev et al., discloses a system for maintaining an electrode between two depths by detecting arc discharges which occur when the electrode depth is outside the range. U.S. Pat. No. 4,483,708, to Gfrerer et al., discloses a system for determining electrode immersion depth based upon the weight of the immersed portion of the electrode, which is determined from the length of the electrode above the slag and the total remaining weight of the electrode. U.S. Pat. No. 4,569,056, to Veil Jr., also discloses a system for controlling melt rate based upon monitoring decreases in weight of the electrode over time. U.S. Pat. No. 4,669,087, to Rasheva et al., discloses another system controlling electrode immersion depth via detection of arc discharges.
It has heretofore been unknown to control electroslag remelting based upon electrical phase difference of the AC circuit in an ESR furnace. The variation in the phase difference of the AC circuit in electroslag remelting suffers from much less variation for a given electrode position than does the voltage for the same position, providing the ability to more accurately control the remelting process and to employ greater immersion depths with the present invention.